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Ion Figuring
ITT's ion figuring system uses an accelerated beam of neutralized argon ions to correct optical surface figure errors rapidly and precisely to the atomic level.
The process provides edge-of-aperture control of the optical surface figure, and eliminates surface deformation and print-through caused by the optical mount or core construction. Highly deterministic and repeatable, ion figuring permits fast and competitively priced production of extremely accurate optical surfaces up to 96in. (2.4m) in diameter. Surface Figure Requirements An optic is ion figured to final specification after conventional or computer controlled small-tool polishing techniques have polished the surface to within a few waves peak-to-valley (p-v) of its final requirement. With ion figuring, optical engineers at ITT routinely demonstrate surface qualities of less than 0.05 waves peak-to-valley (p-v), 0.01 waves rms (0.030 microns p-v, 0.006 microns rms). For the most demanding applications, ITT can achieve surface figures of less than 0.028 waves p-v, 0.006 waves rms (0.018 microns p-v, 0.004 microns rms). High quality surfaces of this type are required for aspheric secondary mirrors, primary mirror segments used in large telescopes, and for microlithography optics used in the semiconductor industry. They are used also to achieve optimum light-gathering and focus properties in aspheric optics for compact digital cameras used on space-based or airborne remote sensing missions. Edge-of-Aperture Control Ion figuring enables ITT to extend the specified aperture to the very edge of the optic. This capability is particularly important in segmented telescope systems where multiple mirror segments form continuous collecting surfaces. Segments with edge-of-aperture profiles minimize light loss, leading to improved optical performance for the overall telescope system. Why does ion figuring make edge-of-aperture control possible? When a mechanical tool's polishing pad overhangs an edge, the lack of support beneath the pad leads to inconsistent and unpredictable material removal rates. With ion figuring, the ion beam applies virtually no physical pressure to the optic. This renders the beam insensitive to geometric anomalies, such as the edge, ensuring a consistent removal profile across the entire surface. Edge-of-aperture profiles play a crucial role in the optical performance of the segmented primary mirrors at the Hobby-Eberly Telescope and W.M Keck Observatory. Mirror segments for both observatories were ion figured by ITT. Mount Distortion Correction Like a scalpel in the hands of a skilled surgeon, ion figuring allows ITT to remove sub-micron-sized bumps and print-through from the surfaces of optics. In lightweight mirrors for space, print-through can occur when the pressure exerted by a honeycomb core distorts the thin mirror surface plate during polishing. This leads to a removal pattern that follows the underlying core pattern. Similarly, mounting points that hold solid mirrors also can deform the mirror surface. ITT has used ion figuring to remove mount distortion from microlithography optics, mirror segments for ground-based observatories, and optics for commercial remote sensing systems. High Determinism Lowers Cost The computer-controlled ion figuring system is highly deterministic, enabling ITT to improve optical surfaces up to a factor of 10 for each iteration of ion figuring and optical testing. A high degree of determinism speeds the finishing process, ensuring specified surface figure within days instead of weeks or months. This reduction in time lowers costs. |
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